R1:315870d56a33

Fix various newline problems in the difference engines

Summary: I'll mark this one up inline since it's all separate bugs.

Test Plan:
- Created a diff with eight changes: (newline absent -> newline present, newline present -> newline absent, newline present -> newline present, newline absent -> newline absent) x (short file with change near end, long file with change near middle).
- Viewed diff in Ignore All, Ignore Most, Ignore Trailing and Show All whitespace modes.
- All 32 results seemed…